1. Field of the Invention
The present invention relates to an organic light emitting device. More particularly, the present invention relates to an organic light emitting device that can increase the lifetime of the device using thereof.
2. Description of the Related Art
Organic light emitting device (“OLED”) is a device that can transfer electric energy to photo energy with high transformation efficiency. The conventional usage of an OLED includes in light guiding devices, display panels, light emitting devices of optical pick-up head and so on. Since the OLED offers a lot of advantages, such as wide viewing angle, fast response speed, wide operation temperature range, less power consumption, full color display, low cost and simple manufacturing process, the OLED has become a major trend in the development of a macromedia display device in recent years.
FIG. 1 is a cross-sectional view illustrating the basic construction of a conventional OLED which includes a substrate 100, an anode layer 102, a hole transport layer 106, an organic light emitting layer 108, an electron transport layer 110, and a cathode layer 114. As shown in FIG. 1, the anode layer 102 is an indium tin oxide (“ITO”) electrode layer, and the cathode layer 114 is a metal electrode layer. When a forward bias is applied to both ends of the electrode layers, the electrons and holes are injected from the metal electrode layer 114 and the ITO electrode layer 102 into the organic light emitting layer 108 respectively. The negative charge carriers (electrons) and the positive charge carriers (holes) will combine in the organic light emitting layer 108 through a radiative combination process, and emit photons. However, the hole mobility is about 2 to 3 orders larger than the electron mobility in conventional OLED. Consequently, in order to balance the mobility of holes and electrons, a hole injection layer 104 is disposed in between the ITO electrode layer 102 and the hole transport layer 106, and an electron injection layer 112 is disposed in between the metal electrode layer 114 and the electron transport layer 110. The formation of the injection layers thus help attain a balanced number of hole carriers and electron carriers.
Nevertheless, even though the number of carriers are balanced, charge will accumulate in the hetero-junction interfaces of the hole transport layer 106 and the electron transport layer 110. As a consequence the physical and electronic properties of the OLED will be degraded, and the lifetime of the device using the OLED will also be decreased.